This invention relates to optical waveguide gratings and, in particular, to etched waveguide gratings particularly useful in add/drop filters, grating-assisted couplers and variable delay lines for optical communication systems.
Optical gratings are important elements for selectively controlling specific wavelengths of light within optical communication systems. Such gratings include Bragg gratings and long period gratings. A grating typically comprises a body of material and a plurality of substantially equally spaced optical grating elements such as index perturbations, slits or grooves.
A typical Bragg grating comprises a length of optical waveguide, including a plurality of perturbations in the index of refraction substantially equally spaced along the waveguide length. These perturbations selectively reflect light of wavelength xcex equal to twice the spacing xcex9 between successive perturbations times the effective refractive index, i.e. xcex=2neffxcex9, where xcex is the vacuum wavelength and neff is the effective refractive index of the fundamental mode. The remaining wavelengths pass essentially unimpeded. Such Bragg gratings have found use in a variety of applications including filtering, adding and dropping optical signal channels, stabilization of semiconductor lasers, reflection of fiber amplifier pump energy, and dispersion compensation.
A difficulty with conventional Bragg gratings is that they filter only a fixed wavelength. Each grating selectively reflects only light in a narrow bandwidth centered around xcex=2neffxcex9. However in many applications, such as wavelength division multiplexing (WDM), it is desirable to have a reconfigurable grating whose wavelength response can be controllably altered.
Long-period grating devices provide wavelength dependent loss and may be used for spectral shaping. A long-period grating couples optical power between two copropagating modes with very low back reflections. A long-period grating typically comprises a length of optical waveguide wherein a plurality of refractive index perturbations are spaced along the waveguide by a periodic distance xcex9xe2x80x2 which is large compared to the wavelength xcex of the transmitted light. In contrast with conventional Bragg gratings, long-period use a periodic spacing xcex9xe2x80x2 which is typically at least 10 times larger than the transmitted wavelength, i.e. xcex9xe2x80x2xe2x89xa710xcex. Typically xcex9xe2x80x2 is in the range 15-1500 micrometers, and the width of a perturbation is in the range ⅕xcex9xe2x80x2 to ⅘xcex9xe2x80x2. In some applications, such as chirped gratings, the spacing Axe2x80x2 can vary along the length of the grating.
Long-period grating devices selectively remove light at specific wavelengths by mode conversion. In contrast with conventional Bragg gratings in which light is reflected and stays in the waveguide core, long-period gratings remove light without reflection, as by converting it from a guided mode to a non-guided mode. (A non-guided mode is a mode which is not confined to the core, but rather, is defined by the entire waveguide structure. Often, the non-guided is a cladding mode). The spacing xcex9xe2x80x2 of the perturbations is chosen to shift transmitted light in the region of a selected peak wavelength xcexp into a non-guided mode, thereby reducing in intensity a band of light centered about xcexp. Alternatively, the spacing xcex9xe2x80x2 can be chosen to shift light from one guided mode to a second guided mode (typically a higher order mode), which is stripped off.
A shortcoming of conventional long-period gratings, however, is their limited ability to dynamically equalize gain. They filter only a fixed wavelength. Each long-period grating with a given periodicity (xcex9xe2x80x2) selectively filters light in a narrow bandwidth centered around. xcexp=(ngxe2x88x92nng).xcex9xe2x80x2, where ng and nng are the effective indices of the core and the cladding modes, respectively. The value of ng is dependent on the core and cladding refractive index while nng is dependent on core, cladding and air indices.
Techniques have been devised for tuning gratings, and an important application of tunable gratings is in the fabrication of tunable add/drop filters in optical communication systems. Tunable filters are widely used in WDM systems to add or drop a channel at the terminals or at an intermediate point in the system. Such filters must have flat passbands and good stopband rejection. UV-photoinduced Bragg gratings written in optical fibers or planar waveguides are typically employed because of their excellent spectral characteristics. Many techniques for fabricating tunable Bragg gratings in fibers have been introduced such as temperature and stretching. When tunable filters are reconfigured in a system, the operation should be transparent to the other channels on the system, i.e. hitless reconfiguration is desirable. This is a limitation for current tunable gratings unless a switch is used to bypass the grating during the reconfiguration period. Alternatively, the grating must have a sufficiently narrow bandwidth to fit in between channels. This requires gratings with extremely good characteristics so that the grating is transparent to adjacent channels.
In accordance with the present invention, a waveguide grating comprises a core and a first cladding material adjacent the core. The first cladding is configured, as by etching, to provide a periodic grating, and a second cladding material having a controllable refractive index overlies the first cladding material. If the index of the second cladding is different from that of the first cladding, the configuration of the first cladding provides an optical grating. If, however, the controllable index of the second cladding is adjusted to equal that of the first cladding, the grating becomes essentially transparent. This grating is particularly useful as a reconfigurable add/drop filter in a WDM optical communication system. It is also useful in grating-assisted couplers and variable optical delay lines.